Stare (Sturnus vulgaris L.) lassen sich auf Himmelsrichtungen dressieren

The environment and the internal clocks: The study of their relationships from prehistoric to modern times

The origin of biological rhythms goes back to the very beginning of life. They are observed in the animal and plant world at all levels of organization, from cells to ecosystems. As early as the 18th century, plant scientists were the first to explain the relationship between flowering cycles and environmental cycles, emphasizing the importance of daily light-dark cycles and the seasons. Our temporal structure is controlled by external and internal rhythmic signals. Light is the main synchronizer of the circadian system, as daily exposure to light entrains our clock over 24 hours, the endogenous period of the circadian system being close to, but not exactly, 24 hours. In 1960, a seminal scientific meeting, the Cold Spring Harbor Symposium on Biological Rhythms, brought together all the biological rhythms scientists of the time, a number of whom are considered the founders of modern chronobiology. All aspects of biological rhythms were addressed, from the properties of circadian rhythms to their practical and ecological aspects. Birth of chronobiology dates from this period, with the definition of its vocabulary and specificities in metabolism, photoperiodism, animal physiology, etc. At around the same time, and right up to the present day, research has focused on melatonin, the circadian neurohormone of the pineal gland, with data on its pattern, metabolism, control by light and clinical applications. However, light has a double face, as it has positive effects as a circadian clock entraining agent, but also deleterious effects, as it can lead to chronodisruption when exposed chronically at night, which can increase the risk of cancer and other diseases. Finally, research over the past few decades has unraveled the anatomical location of circadian clocks and their cellular and molecular mechanisms. This recent research has in turn allowed us to explain how circadian rhythms control physiology and health.

Feasibility of sun and magnetic compass mechanisms in avian long-distance migration

Birds use different compass mechanisms based on celestial (stars, sun, skylight polarization pattern) and geomagnetic cues for orientation. Yet, much remains to be understood how birds actually use these compass mechanisms on their long-distance migratory journeys. Here, we assess in more detail the consequences of using different sun and magnetic compass mechanisms for the resulting bird migration routes during both autumn and spring migration. First, we calculated predicted flight routes to determine which of the compasses mechanisms lead to realistic and feasible migration routes starting at different latitudes during autumn and spring migration. We then compared the adaptive values of the different compass mechanisms by calculating distance ratios in relation to the shortest possible trajectory for three populations of nocturnal passerine migrants: northern wheatear Oenanthe oenanthe, pied flycatcher Ficedula hypoleuca, and willow warbler Phylloscopus trochilus. Finally, we compared the predicted trajectories for different compass strategies with observed routes based on recent light-level geolocation tracking results for five individuals of northern wheatears migrating between Alaska and tropical Africa. We conclude that the feasibility of different compass routes varies greatly with latitude, migratory direction, migration season, and geographic location. Routes following a single compass course throughout the migratory journey are feasible for many bird populations, but the underlying compass mechanisms likely differ between populations. In many cases, however, the birds likely have to reorient once to a few times along the migration route and/or use map information to successfully reach their migratory destination.

Early ant trajectories: spatial behaviour before behaviourism

In the beginning of the twentieth century, when Jacques Loeb's and John Watson's mechanistic view of life started to dominate animal physiology and behavioural biology, several scientists with different academic backgrounds got engaged in studying the wayfinding behaviour of ants. Largely unaffected by the scientific spirit of the time, they worked independently of each other in different countries: in Algeria, Tunisia, Spain, Switzerland and the United States of America. In the current literature on spatial cognition these early ant researchers--Victor Cornetz, Felix Santschi, Charles Turner and Rudolf Brun--are barely mentioned. Moreover, it is virtually unknown that the great neuroanatomist Santiago Ramón y Cajal had also worked on spatial orientation in ants. This general neglect is certainly due to the fact that nearly all these ant researchers were scientific loners, who did their idiosyncratic investigations outside the realm of comparative physiology, neurobiology and the behavioural sciences of the time, and published their results in French, German, and Spanish at rather inaccessible places. Even though one might argue that much of their work resulted in mainly anecdotal evidence, the conceptual approaches of these early ant researchers preempt much of the present-day discussions on spatial representation in animals.

Behavioural and physiological mechanisms of polarized light sensitivity in birds

Polarized light (PL) sensitivity is relatively well studied in a large number of invertebrates and some fish species, but in most other vertebrate classes, including birds, the behavioural and physiological mechanism of PL sensitivity remains one of the big mysteries in sensory biology. Many organisms use the skylight polarization pattern as part of a sun compass for orientation, navigation and in spatial orientation tasks. In birds, the available evidence for an involvement of the skylight polarization pattern in sun-compass orientation is very weak. Instead, cue-conflict and cue-calibration experiments have shown that the skylight polarization pattern near the horizon at sunrise and sunset provides birds with a seasonally and latitudinally independent compass calibration reference. Despite convincing evidence that birds use PL cues for orientation, direct experimental evidence for PL sensitivity is still lacking. Avian double cones have been proposed as putative PL receptors, but detailed anatomical and physiological evidence will be needed to conclusively describe the avian PL receptor. Intriguing parallels between the functional and physiological properties of PL reception and light-dependent magnetoreception could point to a common receptor system.

Experimental and analytical techniques used in bird orientation research

Our present understanding of orientation behaviour in birds is based on a broad array of observational, experimental and analytical (statistical) techniques, which are briefly reviewed here. As an extremely productive model the homing behaviour of pigeons has allowed especially diverse experimental manipulations documenting the involvement of magnetic, visual and olfactory cues in orientation. Work with migratory birds has profited greatly from the design of several kinds of orientation cages, now commonly used, and from hand-rearing test birds under controlled conditions. Free-flying birds, especially on long-distance migration, are still least amenable to study, but radio transmitter technology is providing important new opportunities in this respect. In general, the most valuable studies have been those involving the ontogenetic development of orientation, and those combining several methods of investigation. Some suggestions for further experiments are made.